How can a volcanic eruption be predicted?

Constant monitoring of a volcano is crucial in being able to predict an upcoming eruption. Various technologies are used by volcanic monitoring stations, such as the Hawaiian Volcano Observatory, to detect changes in volcanic behaviour that are signs of an imminent eruption. Seismometers are used to measure increased seismic activity (earthquakes) caused by the movement of magma below, putting pressure on the rock and causing it to crack resulting in earthquakes. Tiltmeters and GPS satellites are used together to detect small changes in the landscape of a volcano. As magma rises it can cause the surface above to bulge from the pressure, leading to changes in the surface height which are detected. For example, before the eruption of Mount St. Helens in 1980, a large bulge grew on the north side of the volcano. Finally, monitoring gas emissions can help predict a volcanic eruption. An increase in the quantity of sulphur dioxide (an eggy smelling gas) and carbon dioxide, is indicative of an eruption as the magma is closer to the surface, so these dissolved gases can escape out from volcanic vents and cracks. The concentrations of these gases can either be measured directly using instruments at the volcanic site or samples of water and air can be collected and analysed in a lab.